Field of the Invention
The present invention relates, in general, to a hybrid vehicle having an electromechanical variable transmission (EVT), and, more particularly, to a power transmission system for a hybrid vehicle, which enables an invariable gear position in a high-speed low-load state using a plurality of planetary gearsets.
Description of the Related Art
Generally, hybrid vehicles are classified into two types: a parallel type and a power split type. The parallel type is divided into a flywheel mounted electric device (FMED) sub-type in which a motor is coupled to an engine and a transmission mounted electric device (TMED) type in which a clutch is coupled between an engine and a motor.
In the FMED type, since the motor is directly connected to the engine, the vehicle cannot run in an electric vehicle (EV) mode, and cannot employ a plug-in hybrid electric vehicle (PHEV) mode in which the vehicle can be charged via an external power source. The TMED sub-type is subjected to a loss of power caused by a change of speed while running in the EV mode, and inevitably employs an electric oil pump (EOP).
In the power split type, a planetary gearset is installed between an engine and a motor, and planetary gears serve as a transmission. In the case of the power split type, an electrical path for controlling a speed of the engine while running in a hybrid electric vehicle (HEV) mode is present during normal operation. The electrical path is a sort of power loss occurring because power from the engine is transmitted to a generator to charge the motor when transmitted from spinning wheels. This electrical path is unfavorable in view of running efficiency and fuel efficiency while running at high speed in the HEV mode.
Meanwhile, Korean Unexamined Patent Application Publication No. 2007-0021445 discloses a hybrid vehicle having a driving system in which a first planetary gearset and a generator are disposed on a first input shaft receiving rotational power from an engine, a motor is disposed on an axis parallel with the first input shaft, and an output from the first planetary gearset is reduced and transmitted to driving wheels through a differential. This hybrid vehicle configuration, however, has a complicated structure and unsatisfactory fuel efficiency.
Therefore, there is a need for a power transmission system for a hybrid vehicle that has a simple structure, supports EV and PHEV modes, allows an engine to be operated at an operating point on an optimum operating line (OOL), provides load leveling, and allows the engine to be directly coupled to a drive unit, all while providing high fuel efficiency while operating at high speed.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.